The pattern specifies gauge of 16 stitches and 18 rows = 4 inches in double-knit on US5 needles using worsted weight yarn. The designer's ravelry page for the scarf (the very one shown on the pattern) says scarf size is 5” by 52”. The pattern is charted and the chart is 224 rows long. When I do the math of her specified gauge, I get that the scarf would only be 49.76" long, not 52". [224 rows / 18 = 12.44; 12.44 x4 = 49.76"]. Is that the correct calculation method? I ask because I thought 52" was going to be a bit short but under 50 would definitely be too short.

Also, I am using worsted weight yarns with the exact wpi (I checked) and even went up two needles sizes to US7 needles (I usually have to go up 1 size to get gauge) and I am only getting 3 inches per 18 rows. At this gauge, my scarf is only going to be 37.32" long! If I go up to larger needles I am afraid you'll be able to see the contrasting color through the stitches because the stitches will be so large. I thought about trying to repeat part of the pattern on each side of the scarf but figuring out what section(s) to repeat so as to keep the flow of the trailing hearts and the spacing and end up with the right length...ugh!! I already have about 8" made so any repeats would have to be made after row 52 and before row 172. I really don't want to frog it! This was supposed to be the "easy" Christmas present knit. I have never had anything so "off gauge" before. *sigh* I should have swatched but I thought "it's just a scarf".

Your math looks right. Does the pattern have any rows before and after the charted part, i.e., a border? As for your gauge, you've just made an 8" swatch, and it's shown that the gauge is off. It happens. Is your yarn a true worsted weight? What gauge does the ball band list? Yarn weights aren't exact, so you might have a worsted that's on the thin side. If you maintain the density you need, do you have enough yarn to do an extra repeat, i.e., an additional 18 rows?

It's one of the lovliest double knit projects. You're figuring out the length correctly and what matters more, the length that you'll get at your gauge. In orfer to make the scarf 52" instead of 37" you'd need 90 extra rows which you could add at the center by repeating the heart at the center of the scarf or some part of the other charts (as you've suggested). Alternatively, you could go with the 37" or a little longer (maybe 3 repeats total of the center heart). It'll be a nice warm scarf, just not the double wrap around kind.

After some rest, I have decided to keep moving forward with the scarf. To get the length I want, I will repeat the center heart 4 times as that is the easiest to do without interrupting the pattern. Thanks!